settings: add metrics settings extension

[sumukhballal]

Issue number:

Closes #3656

Description of changes:

Creates a metrics settings extension and uses it in every variant's settings model.

Testing done:

Built an aws-ecs-1 variant with the settings-metrics package installed. Called apiclient to verify that the aws settings worked as before. Also called the settings extension to verify that it was behaving as expected.

$ apiclient get settings.metrics
{
  "settings": {
    "metrics": {
      "metrics-url": "https://metrics.bottlerocket.aws/v1/metrics",
      "send-metrics": true,
      "service-checks": [
        "apiserver",
        "chronyd",
        "containerd",
        "host-containerd",
        "docker",
        "ecs"
      ]
    }
  }
}

Setting the URL: Happy case

[ssm-user@control]$ apiclient set settings.metrics.metrics-url="https://metrics.bottlerocket.aws/v2/metrics-test"
[ssm-user@control]$ apiclient get settings.metrics.metrics-url
{
  "settings": {
    "metrics": {
      "metrics-url": "https://metrics.bottlerocket.aws/v2/metrics-test"
    }
  }
}

Setting the service-checks

[ssm-user@control]$ apiclient set --json '{"settings": {"metrics": { "service-checks": ["test", "test2"]}}}'
[ssm-user@control]$
[ssm-user@control]$
[ssm-user@control]$ apiclient get settings.metrics.service-checks
{
  "settings": {
    "metrics": {
      "service-checks": [
        "test",
        "test2"
      ]
    }
  }
}

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[sumukhballal]

Addressed comments from @sam-berning !

[webern]

You need to run cd sources && cargo check (it may error, but it's ok) then check in the Cargo.lock changes.

[sumukhballal]

Added changes to address @webern's comments.

[sumukhballal]

Fixed some rustfmt issues.

[sumukhballal]

Fixed merge conflicts.